Many types of filter materials have been proposed for decreasing the amount of certain ingredients of tobacco smoke reaching a smoker's respiratory system. When used for filtering tobacco smoke, the filter material should not distort the taste of the smoke by adding a taste of its own and should be capable of inexpensive fabrication so as not to make the ultimate price of the smoking article too costly.
While a wide variety of fibrous materials have been employed as filter material, only paper and cellulose acetate filters have met with any significant commercial acceptance. Paper filters are usually corrugated and condensed into a rod form for attachment to a cigarette. Unfortunately, they tend to adversely affect the taste and odor of the delivered smoke stream and, due to their high moisture absorbency, tend to collapse during use since the compressibility of moist paper filters at a given pressure drop is generally greater than other conventionally used filters of comparable weight.
Cellulose acetate is conventionally used in the form of a tow of continuous filaments. These filters overcome all the aforementioned disadvantages of paper filters while admirably meeting the requirements of good draw and economy. As a result, a major portion of filter cigarettes utilize this type of material in spite of the fact that tow filters exhibit smoke removal efficiencies at a given draw that are relatively lower than that of paper filters.
An alternative method of utilizing cellulose acetate is the formation of nonwoven webs, or felted batts from staple fibers. Such fibrous structures lack dimensional stability and necessitate the use of binders to maintain the fibers in their desired array.
It also has been suggested in U.S. Pat. Nos. 4,192,838, 4,274,914 and 4,283,186 that cellulose acetate fibrets may be effective to hold the cellulose acetate fibers in the absence of a binder and still provide the desired high surface area for filtration. The patents indicate the use of binders decreases the surface area available for filtration, adds an undesirable taste to the filtered smoke and represent a limiting factor in the speed of filter manufacture because of the time necessary to attain complete bonding. However, the binder free materials have a tendency to break when run through the corrugating and plug forming machinery and as a result, the components of the web are not securely anchored within the webs, exhibiting a significant solid particulate or "dusting" problem.